A GFCI does its job and can save lives when used properly, it only takes like 5 milliamps of current flow out the Hot Line thats NOT being returned via the GrounDED Conductor Neutral (i.e. a leak) to trip them out. If you run the Hot and Neutral through a Torroidal Coil the currents should balance out so thers no induced voltage, but if some of the return is flowing elsewhere, a voltage gets induced and the GFCI
Is it common for lights to be used on a GFCI you ask?? NOT if it was my design, but yes Ive seen it
Its been a while but when I was a design engineer my practice was:
I NEVER protected pure ceiling lighting only branch circuits with a GFCI.
I NEVER mixed and matched light circuits and convenience outlet circuits i.e. light circuits had lights ONLY and outlet circuits had outlets ONLY not lights PLUS Outlets all on the same circuit. I kinda figured if you were working in a dark room and tripped the breaker on the outlet circuit you sure wouldnt WANT THE LIGHTS TO GO OUT ALSO !!!
I, of course, used GFCI where the NEC required them such as within x inches of sinks and lavatories and outdoor and garage circuits etc unless they were dedicated for say a freezer and there I used SINGLE outlets in not readily accessible locations (say hidden away behind the freezer).
Unless theres leakage current (or perhaps excess inductance) A GFCI shouldnt necessarily trip because a fluorescent light is used. HOWEVER electricity, inductance and some electronic switching can do strange things in a GFCI so I never say never lol
Similar, I wouldnt expect the use of a meter to trip a GFCI. They draw extremely lowwwwwwwww current and it all should be returned via the Neutral
Id guess theres still a hidden GFCI tripped somewhere in a closet or elsewhere that feeds lights downsteam thats the problem, but finding it?? Maybe a proximity type meter to trace down the power or loss thereof.
Like I say its been a while for me, maybe some of the more current practicing electricians have more ideas???
Ol John T and all